This invention relates generally to monochromators, and more specifically concerns a double pass grating monochromator which eliminates the need to optically chop at the intermediate slit to prevent spectral overlap, while simultaneously preserving the advantages of invariant slit image curvature and matched Lagrange constants at each pass. The subtractive dispersion arrangement and the selective scattering properties of reflection diffraction gratings results in a monochromator with a second stage which functions to give additional optical filtering but no bandwidth narrowing.
In utilizing grating monochromators as optical filters for spectral instrumentation, it is often desirable to gain additional filtering by arranging the optics so that the light makes more than one pass on the grating. This is usually done by interposing into the beam near the intermediate slit a system of two or more small flat mirrors which function to return the beam for an additional pass on the grating. The advantage of such a double passing scheme over simply adding a second complete monochromator stage, is one of savings of cost and space. Various prior-art schemes for double passing gratings have been devised. If one ignores, for a moment, some of the obvious common problems such as overlapping spectra, back or other order diffraction, and general scatter, these can best be viewed in context with the invention by considering the following three characteristics:
1. Match of slit image and slit curvature as a function of grating angle (or wavelength); PA1 2. Match of Lagrange invariant (see p. 43 of "Applied Optics and Optical Design," A. E. Conrady, Oxford University Press, 1943) on the first and second grating passes; and PA1 3. Dispersion -- normally additive (but may be subtractive). PA1 .PSI. = arc sin S/R, and is substantially less than 90.degree., PA1 S = 1/2 the length of the line extending between the entrance and exit slits, and PA1 R = the distance between the intersection and either of the entrance and exit slits.
To elaborate briefly, failure to satisfy condition (1) results in loss of resolution, or of energy per spectral band; failure to satisfy condition (2) results in direct loss in energy; and finally, if one fails to arrange the optics so that the second and first pass dispersions are adding condition (3), the entrance and intermediate slits must be narrowed appropriately to pass the desired optical bandwidth, resulting in a corresponding reduced thruput as compared to the additive arrangement. In the subtractive arrangement, since the resolution and dispersion are determined by one pass alone, the entrance and intermediate slits or the intermediate and exit slits, must be carefully matched, controlled and tracked. The "non-resolving" pass functions only to gain additional reduction in background by dispersing again those spurious wavelengths which are scattered during the resolution-controlling pass.
It is a prime purpose of known prior art schemes to gain additional dispersion. Hence condition (3) is satisfied along with either (1) or (2). For example, in U.S. Pat. No. 3,454,339, the arrangement satisfies conditions (3) and (1). In that system, the entrance and exit slits are together offset vertically below the horizontal Ebert plane, and horizontally displaced one on each side of the grating. A pair of perpendicular intermediate flat mirrors, whose line of intersection is parallel to the grating rulings, are inserted in the vicinity of the intermediate slit, one generally above the entrance and one generally above the exit slit. Because the beam is passed directly behind the grating, it is necessary to move the grating a considerable distance inside the collimator focal plane. This, together with the fact that the grating diffraction angle on pass one and incidence angle on pass two are appreciably different, results in sizable Lagrange mismatch or grating overfill which is a function of wavelength.
In addition to the above problem, another inherent difficulty exists in the scheme of the above patent. It is possible for a particular order of stray radiation to find a reverse path through the intermediate slit and a different order of the same radiation to find a path directly through the exit slit after a second pass on the grating. In some cases this stray radiation can lie within the bandpass of normal order-sorting filters. This problem has been observed experimentally and reported in the technical literature (N. R. Butler, Applied Optics, Vol. 9, No. 6, 1475, June, 1970). As will be shown later, both of the above mentioned problems are solved in the present invention.
Another double passing scheme is described in the Fastie-Sinton U.S. Pat. No. 2,922,331. In this arrangement horizontal beam displacement and image inversion takes place near the intermediate slit image by means of a pair of perpendicular corner mirrors whose line of intersection is parallel to the grating rulings. The spectral overlap problem is solved by means of a pair of additional parallel mirrors which function to cause a vertical displacement in the vicinity of the intermediate slit image. Thus, the second pass is essentially out-of-plane with respect to the grating and incident from the opposite side as the first pass. In this way, additive dispersion is accomplished as well as a good match of Lagrange constant. However, the inversion that provided for additive dispersion also resulted in the inverting of the image curvature in such a way that condition (1), that of invariant slit image curvature with wavelength, is not satisfied. Because of this, the utility of the arrangement is limited by the necessity of either using rather short slits and thus lower thruput, or scanning limited wavelength regions.